Page 147 - Power Quality in Electrical Systems

Chapter
9

Uninterruptible Power

Supplies

The term Uninterruptible Power Supplies (UPS) has been applied
both to an uninterruptible power system and to the specific
battery-inverter equipment incorporated into the system. In this
chapter, we will use the term UPS for the specific equipment.
The UPS was developed in parallel with digital computers
and other IT equipment to provide a reliable uninterruptible
electric power source to create a standard the electric-utility
industry could not provide. The computer industry would not
have developed without some type of UPS, which either used
engine-generator sets or static inverters employing power
electronic devices. If the concept of an independent UPS had
not been developed, then all electronic equipment sensitive to
disturbances in source voltage would have had to incorporate
energy storage means—for example, batteries to counteract
such disturbances in the source voltage.

Introduction
The concept of a double-conversion UPS is shown in Figure 9.1a [9.1].
The essential parts are
■ The inverter, for converting power from DC to AC
■ The battery, to supply power to the inverter when AC power is
interrupted
■ The converter-battery charger, to supply DC power to the inverter,
as well as to charge the battery
■ The by-pass circuit, to supply AC power directly to the load when
the inverter fails. A second by-pass circuit external to the UPS—the

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